1. Field of the Invention
This invention relates to a water-tight watch case using an inorganic glass crystal and particularly to a construction for preventing fogging of the crystal due to vapor formation inside the case when it is used under severe conditions.
2. Prior Art
In a conventional water-tight case, its glass crystal sometimes becomes fogged with condensed vapor inside the case when the wristwatch is subjected to a sudden temperature change even if no external vapor permeates into the case. The reason for this is that some vapor is always included in the air and it is actually impossible to prevent some permeation of vapor into the case when it is in an atmosphere of high humidity.
In recent years, inorganic glass has become increasingly utilized as the crystal for timepieces. Such inorganic glass material has a larger thermal conductivity than conventional organic glass material such as acrylic resin. Therefore, if an inorganic glass crystal and an organic glass crystal are cooled under the same conditions, the former is more susceptible of fogging than the latter because the temperature becomes lower inside the former than inside the latter.
Although this fogging has no influence on the watch movement, it can cause apprehension to its user. Moreover, in the case of a divers' watch or the like, the fogging may cause a serious accident if it prevents the user from reading its time indication.
It is known to coat the entire inner surface of an inorganic glass crystal with a water-absorptive material to prevent condensed vapor from collecting and fogging the inner surface of the glass crystal. However, in the known construction, water can be absorbed by the water-absorptive material and gradually permeate into the case.